I. Field of the Invention
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved method and apparatus for paging a cellular telephone or other wireless communications device.
II. Description of the Related Art
The IS-95 cellular telephone standard (and its derivatives such as IS-95A and ANSI J-STD-008 referred to herein collectively as IS-95) uses advanced signal processing techniques to provide efficient and high quality cellular telephone service. For example, an IS-95 compliant cellular telephone system utilizes vocoding, error detection, forward error correction (FEC), interleaving and spread spectrum modulation in order to make more efficient use of the available RF bandwidth, and to provide more robust connections. In general, the benefits provided by IS-95 include longer talk time, higher capacity, and fewer dropped calls when compared to other types of cellular telephone systems.
To conduct communications in an orderly manner, IS-95 provides a set of highly encoded channels over which data having different functions is transmitted. These highly encoded channels include a paging channel over which paging messages are transmitted notifying cellular telephones or other types of wireless terminals that an incoming request to communicate is pending. In accordance with the IS-95 standard, paging messages are transmitted at low to medium data rates (4800 or 9600 bps) during time slots that are pre-assigned to groups of cellular telephones. Table I provides the data included in a General Page Message as an example of a typical paging message generated substantially in accordance with the IS-95A standard.
Table I is provided simply to illustrate the length of a typical paging message, so a detailed description of the function of each field is not included herein. Such a detailed description may be obtained, however, by referring to the well known, and publicly available, IS-95 standard (in particular the IS-95A standard). The paging messages also begin with an eight bit message length field (MSG_LEN), that indicates the length of the message, and end with a 30 bit cyclical-redundancy-check (CRC) field (not shown).
To monitor for paging messages, a cellular telephone periodically monitors the paging channel during the assigned paging slot. In particular, the cellular telephone periodically activates complex RF and digital signal processing circuitry for as long as is necessary to successfully process the paging message. Since the typical paging message is relatively long, and transmitted via a highly encoded low to medium rate channel, the associated processing during each paging slot requires a significant amount of time and signal processing resources, and therefore requires a significant amount of power to complete. This reduces the amount of time an IS-95 cellular telephone can remain in standby mode using a battery of given capacity, and therefore is highly undesirable.
The present invention is a novel and improved method and system for paging a cellular telephone or other wireless terminal that reduces standby mode power consumption. In accordance with one embodiment of the invention, a minimally encoded quick paging channel is established over which short, quick page messages are transmitted during one of a set of quick paging slots. The quick page message indicates that a request to communicate has been received and that the receiving communication terminals should process a highly encoded full paging channel transmitted during the next full paging slot for a more detailed, full page message. A communications terminal monitors full paging channel after a quick page message has been received on the quick paging channel.
To page the communications terminal, a base station controller first generates the quick page message during a quick page slot assigned to a set of communications terminal that includes the particular communications terminals being paged. This is followed by a full page message identifying the particular communications terminal. The communications terminal periodically monitors the quick paging slot and, upon detecting the quick page, activates decoding circuitry for processing the full paging channel. Upon processing the full paging channel, the communications terminal determines if the full page message is directed to it, and if not, deactivates the decoding circuitry and returns to processing the quick paging channel.
Since the Quick Paging Channel carries only indicator bits, and no messages with CRC, a direct supervision of the Quick Paging Channel is not feasible as with that of the Paging Channel or the Traffic Channel. However, a loss of the Quick Paging Channel due to either base station transmission error or mobile station detection error will cause missed pages and, as a result, affect the service quality. A method to indirectly supervising the Quick Paging Channel is proposed below to mitigate this problem.
In the present invention, using the quick paging channel can result in one of four possible outcomesxe2x80x94a false alarm, a miss, a hit or an erasure. A false alarm is an event in which the Quick Paging Channel indicates there is a page addressed to the mobile station while no page is received in its assigned Paging Channel slot (with the Paging Channel message received correctly). A miss is an event in which the Quick Paging Channel indicates there is no page addressed to the mobile station while a page is received in its assigned Paging Channel slot. A hit is an event in which the Quick Paging Channel indicates there is a page addressed to the mobile station while a page is indeed received in its assigned Paging Channel slot. In the exemplary embodiment, the Quick Paging Channel is transmitted at a fixed transmission energy relation to the pilot channel transmitted by base station 12. The relationship of the quick paging channel energy to the pilot channel is known by mobile station 10. When the energy of the pilot channel received by mobile station 10 is below a predetermined level, mobile station 10 determines that detection of the quick paging channel is not reliable and declares an erasure.
A mobile station operating in the slotted mode, while monitoring the Quick Paging Channel, shall monitor the Paging Channel in its assigned slot at least once every N minutes, regardless of what the Quick Paging Channel indicates for that slot. An example value for N is 10. The N-minute windows in time can be disjoint fixed windows next to each other. An alternative is a moving window that moves with the slot cycle. Another alternative is a moving window with its start reset when a hit occurs.
In the present invention, mobile station 10 keeps a first counter for the number of false alarms that have occurred (QPCH_FA), a second counter for the number of misses that have occurred (QPCH_MI), a third counter for the number of hits that have occurred (QPCH_HI), a fourth counter for the number of erasures that have occurred (QPCH_ER) and a fifth counter for the total number of Quick Paging Channel messages received in the time interval (QPCH_TO). The values recorded in these counters are stored by the mobile station and will be sent to the base station when so requested. These counters can be reset at powering-up or by the base station via messaging, or both. When the number of accumulated misses in a given time interval T1 exceeds a certain threshold Nm1, the mobile station may send a message to inform the base station of this. When the number of accumulated misses in a given time interval T2 exceeds a threshold Nm2, the mobile station shall monitor the Paging Channel in its assigned slots regardless of what the Quick Paging Channel indicates. T2 and Nm2 could be the same as or different from T1 and Nm1, respectively. The mobile station may return to follow the Quick Paging Channel indication as to whether to monitor the Paging Channel or not when the number of accumulated misses in a given time interval T3 drops below a certain threshold Nm3, or when Nh1 hits occur in a row, or both.